The roof structure of an industrial building typically comprises rafter beams which extend parallel to one another across the building and slope from the center of the building down toward its sides, and purlins which extend parallel to one another and which extend across and are mounted on the rafter beams. Hard sheets of exterior roofing material extend over and are attached to the purlins. In the past, when a roof structure of an industrial building was to be insulated, elongated sheets of insulation material were stretched across the purlins and the sheets of hard roofing material were attached to the purlins through the insulation material. The relatively thin sheets of insulation material were applied to the roof structure by the workmen using the sheets of hard roofing material which were already installed in the roof structure as a working surface. Reels of insulation material were first unwound on the hard sheets of roofing material and the long sheets of insulation material were moved by hand over onto the exposed purlins adjacent the hard roofing material and the lengths of the sheets of insulation material extended across the lengths of the purlins. The sheets of insulation material were stretched to prevent sagging between the purlins, and the hard roofing material was then placed over the insulation material and connected to the purlins.
As set forth in my prior U.S. Pat. No. 3,559,914, it has now become common practice to extend the sheets of insulation material along the lengths of the purlins, instead of across the purlins, so as to eliminate the seams between adjacent sheets of insulation material from being exposed inside the building. The new procedure as set forth in my patent has reduced hazards to workmen on the roof by maintaining the reels of insulation material in a relatively static and available position on the exposed purlins without exposing long lengths of a sheet of material to the wind while the workmen remain on the sheets of hard roofing material, so that the occasions when the workmen might be tempted to walk or climb out on the purlins to place or adjust the sheets of insulation material have been reduced.